1. Field of the Invention
The present invention relates to a method of controlling a backlight module, a backlight controller and a display device using the same, and more particularly, to adjust the backlights according to the image content in spatial domain and in temporal domain.
2. Description of Related Art
With great advance in the techniques of electro-optical and semiconductor devices, flat panel displays, such as liquid crystal displays (LCD), have enjoyed burgeoning development and flourished in recent year. Due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. An LCD includes a liquid display panel and a backlight module. The liquid display panel has no capacity of emitting light by itself so that the backlight module is arranged below the liquid display panel to provide the surface light source for the liquid display panel so as to perform the display function.
Generally, a cold cathode fluorescent lamp (CCFL) is disposed on the backlight module and is used for providing a white light as the backlight. Nevertheless, the luminance of the white light depends on the dimension of the CCFL and it is difficult to design a light, thin and small LCD by using the CCFL. Owing to the advantages of the light emitting diodes (LEDs), such as smaller size, lower cost, higher luminance, and higher operational life than the CCFL, the LED gradually replaces the CCFL to provide the backlight.
As for the LED backlight, there are two methods to produce the white light. The first method is to use a white light LED manufactured from a blue LED spread with a fluorescent powder. The white light is produced when the fluorescent powder is excited by the blue light, and the white lights with different color temperatures can be produced by using different fluorescent powders. However, the light-emitting efficiency the white light LED is lower when initially operated and then would cause the luminance of display screen lower. Moreover, the white light often approaches to a blue or a green wave band and does not match the color filters.
The second method is to utilize the red, green and blue LEDs to mix for various color backlights including the white light. Since the wavelengths of the red, green and blue LEDs are close to the transmission wave peaks of the color filters, the color rendering of the mixed white light can be exceed more than 100% of the standards stipulated by National Television Standards Committee (NTSC). Furthermore, the color temperature of the mixed white light can be freely adjusted by regulating the luminance of light emitted from the red, green or blue LEDs.
In general, if equal luminance of each of the backlights from the red, green and blue LEDs is provided to the liquid display panel, the displaying quality of the image changes as the image content. Hence, there are some people previously mentioned that adjusting the backlights according to the local image content can improve the problem of unstable displaying quality, that is, respectively determining the luminance of each of the backlights according to the luminance distribution of each image block. However, adjusting the backlights by the image blocks destroys the spatial continuity of the image, and thereby human eyes perceive non-uniform area between the image blocks to affect the display of color saturation and luminance of the image. Besides, human eyes also perceive flickers when displaying continuous images (or called video) in temporal domain and the flickers cause human eyes get tired easily.
As the above-mentioned description, a low pass filter (LPF) used for adjusting the backlights is needed to smooth the non-uniform area or reduce flickers perceived by human eyes in the images. Since a response of a fixed LPF may be over fast or slow with regard to different image blocks, how to design a proper LPF adapting to the image content for adjusting the backlights becomes an important issue to be researched and discussed.